1. Field of the Invention
This invention relates to an improved mechanical dewatering process wherein high moisture content organic carbonaceoss materials such as peat, agricultural and municipal waste, sewage sludge, and biomass, are mixed with additive particulates and then mechanically dewatered to yield organic carbonaceous product having a moisture content of less than about 50 weight percent total water which is suitable for use directly as a fuel, as substrate for subsequent conversion processes, or, in the case of peat, for horticultural purposes.
2. Description of the Prior Art
Abundant, renewable and inexpensive organic carbonaceous raw materials such as agricultural and municipal wastes, sewage sludge, and biomass, and abundant but non-renewable raw materials such as peat, may be dewatered to provide fuel directly and to provide raw material for conversion processes to produce gaseous and liquid energy sources. These organic carbonaceous raw materials, in their naturally occurring states, contain substantial amounts of water. For example, raw peat may comprise greater than 90 weight percent total water. Significant amounts of water must be removed from these raw organic carbonaceous materials before they can be utilized as energy sources. Dewatering techniques known and practiced in the art include mechanical means, thermal drying, and air drying utilizing solar energy.
Solar energy may be utilized to achieve a product moisture content of less than about 50 weight percent when organic carbonaceous material is air dried. Most commercially processed peat is air dried. While this dewatering process is energy efficient, it requires setting aside a large land area for an extended time period, and it is entirely weather dependent. Weather conditions, particularly in the northern regions, are too severe and utilization of solar energy for air drying is not commercially practical.
Mechanical dewatering means, such as roll and belt presses, filter presses, screw presses, and centrifuges, are presently utilized to reduce product moisture content to about 65 to 75 weight percent water. Mechanically dewatered organic carbonaceous material ordinarily must be further thermally dried to achieve the desired product moisture content, usually less than 50 weight percent moisture. Thermal drying is accomplished, typically, by means of a rotary drum, fluidized bed, or flash drier. All of the thermal drying techniques require substantial energy inputs, thereby rendering the technique of mechanical dewatering in combination with thermal drying inefficient and expensive.
Wet carbonization processes are known to the art and are utilized primarily for beneficiation, but wet carbonization also improves the dewaterability of high moisture content organic material. Typically, wet carbonization entails heating the organic carbonaceous material to temperatures between about 300.degree. and 650.degree. F. at pressures between about 300 and 2500 psi, for residence times of about one hour or less. Chemical decarboxylation and dehydration occur during wet carbonization to permit more effective mechanical dewatering, and to enhance the heating value of the dewatered solids. Several wet carbonization processes are reviewed in Mensinger, "Wet Carbonization of Peat: State-of-the-Art Review", Peat as an Energy Alternative Symposium Papers, published by Institute of Gas Technology, September 1981, pp. 249-280. All of the systems reviewed in this paper employ water slurry treatments requiring high temperatures and elevated pressures.
Several types of dewatering systems are reviewed and evaluated in Tsaros, "Peat Dewatering, An Overview", paper presented at the IGT symposium Peat as an Enerqy Alternative II, and published by Institute of Gas Technology, June 1982, pp. 199-216. Comparative costs for four different systems utilizing mechanical dewatering in combination with thermal drying to achieve product moisture contents of less than 35 weight percent are presented in Tsaros, "Comparison of Dewatering Costs", paper presented at the IGT symposium Peat as an Energy Alternative II, and published by Institute of Gas Technology, June 1982, pp. 253-281.
Significant quantities of municipal sludge and manufacturing by-product wastes are generated yearly in the United States. Appropriate methods for dewatering these wastes for proper disposal vary with the type of waste material and with the ultimate use. For example, wastes disposed by incineration may require a lower moisture content than wastes disposed by landfill.
Product moisture contents of dewatered sludges vary considerably depending upon the dewatering process, the type of sludge, and the type and amount of coagulant or additive used. Drying beds are the most widely used method of municipal sludge dewatering in the United States, and, depending upon the type of sludge, the processing rate required, and the degree of dryness required, this method can produce product moisture contents of 50 to 60 weight percent. Extending the drying period beyond the typical three weeks can reduce the moisture content even further. More than one-half of the municipal sludge produced in the United States is dewatered by drying in this manner.
Despite the low capital cost and low product moisture contents achievable, the use of drying beds for sludge has certain disadvantages: the sludge must be stabilized; more land is required than is required for fully mechanical methods; and the system must be designed with careful concern for climatic conditions. In addition, sludge drying beds are highly visible to the public, they are subject to periodic odors, and the removal of dried sludge is labor-intensive.
The product moisture contents of sludge dewatered by various methods are presented in Table 1, below. These values represent the lowest values reported for dewatered digested primary sludge, waste-activated sludge (WAS), and mixtures of the two, achieved with or without ferric chloride, lime or ash addition. The data show that pressure filtration achieves as low a product moisture content as drying beds (50 to 60 weight percent). However, pressure filtration is an expensive process with higher capital requirements and operating costs than the other methods listed.
TABLE 1 ______________________________________ RANGE OF MOISTURE CONTENTS OF DIFFERENT SLUDGES DEWATERED BY VARIOUS METHODS.sup.+ Dewatered Sludge Type Digested Digested Primary Primary WAS* Plus WAS wt. percent moisture** ______________________________________ Gravity 88 97 to 98 92 Thickening Belt Pressing 80 to 84 77 to 82 -- Centrifugation 65 to 72 90 to 92 70 to 85 Vacuum 62 to 65 85 78 to 86 Filtration Pressure 60 50 to 55 50 to 60 Filtration Drying Bed 50 to 60 (Sand) ______________________________________ *Waste Activated Sludge (WAS) **Lowest moisture achieved is listed .sup.+ Ettlich, W. F., et al, Sludge Handling and Conditioning. EPA Repor No. EPA 430/978-002, February 1978
Addition of coal fines to sewage sludge as a filter aid, coagulant, or deodorant, is known. U.S. Pat. No. 4,159,684 discloses a process wherein coal fines are added to sewage sludge prior to or during dewatering to enhance filtration. Coal fines are consumed in the process, and the chemical properties of the sludge substrate are altered. Fine coal or lignite dust is added to concentrated sewage sludge to coagulate and concentrate solids, thereby enhancing vacuum filtration, in the process taught by U.S. Pat. No. 3,933,634. Again, coal fines are incorporated into the product reducing the weight percentage moisture and improving the calorific value and are consumed as part of the product. U.S. Pat. No. 3,300,403 teaches the addition of powdered coal to sewage sludge as a deodorant, and to enhance the sedimentation of solids. The powdered coal does not significantly reduce the moisture content of the sludge solids, but only that of the mixture due to addition of dry powdered coal which is present in the product, and enhances subsequent combustion.